An organic light-emitting diode device, also called an OLED device, commonly includes a substrate, an anode, a hole-transporting layer made of an organic compound, an organic luminescent layer with suitable dopants, an organic electron-transporting layer, and a cathode. OLED devices are attractive because of their low driving voltage, high luminance, wide-angle viewing and capability for full-color flat emission displays. Tang et al. described this multilayer OLED device in their U.S. Pat. Nos. 4,769,292 and 4,885,211.
In a bottom-emitting type of display where light is emitted downward through the substrate, the overall area that can emit light is limited by the presence on the substrate of circuitry such as Thin Film Transistors (TFT's), which are opaque. Consequently the open area available for the light to emerge is reduced. In top-emitting device structures where the light is made to emerge primarily through the top surface away from the substrate and TFT circuitry, the emission area can be significantly higher than the conventional bottom-emitting devices. Therefore, much work has been done to produce OLED displays which are top- or surface-emitting. This configuration has potential to improve display performance compared with bottom-emitting OLEDs.
A common problem with OLED displays is sensitivity to water. Typical electronic devices require humidity levels in a range of about 2500 to below 5000 parts per million (ppm) to prevent premature degradation of device performance within a specified operating and/or storage life of the device. Control of the environment to this range of humidity levels within a packaged device is typically achieved by encapsulating the device or by sealing the device and a desiccant within a cover. Desiccants such as, for example, molecular sieve materials, silica gel materials, and materials commonly referred to as Drierite materials, are used to maintain the humidity level within the above range. Particular highly moisture-sensitive electronic devices, for example, organic light-emitting devices (OLED) or panels, require humidity control to levels below about 1000 ppm. This necessitates sealing an OLED display to prevent the incursion of moisture, for instance by a hermetic seal around the outside of an OLED display. However, hermetic sealing is extremely difficult to achieve or expensive to manufacture, while nearly hermetic sealing is often insufficient in such moisture-sensitive devices, which often require the addition of highly active moisture-absorbing materials.
Numerous publications describe methods and/or materials for controlling humidity levels within enclosed or encapsulated electronic devices. Kawami et al., in U.S. Pat. No. 5,882,761, has taught the use of a desiccant layer over the organic layers of an OLED display, between the substrate and the top seal. Kawami et al. teach the use of the following desiccants: alkali metal oxides, alkali earth metal oxides, sulfates, metal halides, and perchlorates. Such materials can be deposited in a predetermined shape by such techniques as vacuum vapor deposition, sputtering, or spin-coating. Boroson et al., in U.S. Pat. No. 6,226,890, disclose the use of a castable blend of the above desiccants with a suitable binder. However, many desiccating agents can be reactive toward the layers and electrodes of OLED devices, and a number of ways have been proposed to keep the desiccating agents from contacting the OLED components. Kawami et al., in the '761 patent, have taught that the drying agent is to be coated on the inside surface of an airtight container. Boroson et al., in the '890 patent, use the castable blend to coat the interior surface of an enclosure. Techniques such as these require additional materials and efforts.
In addition, arrangements described in the above patents may necessitate placing an opaque layer over the top of the organic layers of the device, including the emitting layer. This arrangement is acceptable for bottom-emitting devices, but it cannot be used for a top-emitting OLED display, as the emission of light would be partially or completely blocked.
Tsuruoka et al., in U.S. Patent Application Publication 2003/0110981, have disclosed a series of transparent drying agents which operate by chemisorption and can be used in an OLED display. These are conceived as useful in OLED devices wherein one wishes to allow light emission through a desiccant layer. However, a desiccant—especially a chemisorption desiccant—is designed to change in the presence of moisture. Therefore, it is possible that the properties of the optical path of the device will change during the device lifetime, leading to potential visual changes in the display. This can limit the usefulness of this method.
One potential method of avoiding the above problems in a top-emitting display has been to put a ring of desiccant around the perimeter of the organic layers. This places the desiccant in a very useful position, close to the outer seal of the device, and any moisture passing the outer seal will first encounter the desiccant ring. However, this technique has the undesirable side-effect of increasing the size of the device without providing any added display area, or limiting the amount of desiccant to minimize the added display area. Thus, a top-emitting device using this technique can be significantly larger or contain significantly less desiccant than the equivalent bottom-emitting device. For some applications, this can be unacceptable.